thesims.ea.com -- The official site.

The Sims Resource -- One of the biggest Sims sites around. Some of the Sims 2 and 3 content is pay, but all Sims 1 content and the forums are free.

Simblesse Oblige -- A good Sims 1 forum.

SimFreaks -- Another good site. Sims 1 content is now free.

SimSlice (pay) -- Lots of great hacked objects here.

The SiMstitution -- Lots of great hacked objects here too. Now closed, but the Wayback Machine has a copy of much of it.

7 Deadly Sims -- Another good site, with lots of good objects, skins, walls, and floors.

The 8th Deadly Sim (pay) -- A paysite, run by the same people who run 7 Deadly Sims. Has some good content.

Paladin's Place -- The home of SimWardrobe, and several other great add-on programs for The Sims.

SimEnhancer -- The best program around for changing sims' heads, skins, and other attributes. Free 48-hour trial period, $17.95 US thereafter.

Sims Finder -- A searchable index of Sims 1 objects, with links to the sites they're on. Run by OddSim.

The Ultimate Sims List -- A listing of a very large number of Sims sites. You can also vote for sites listed there -- click here to vote for mine.